Separation of olefins



* v stares Patented May 14, 1946 PATENT- OFFICE J w h r 2,400,350 I i a. SEPARATION or OLEFINS' 7 use p. mu. Baytown, Tea. assignor to Standard Oil Development Company, a corporation of Delaware Application Au usta, 1944, semi M54801: scum. ((21. 200-677) FThepresent invention is directed 'to a method forfrecovering relatively pure normal butylenes from a hydrocarbon C4 fraction I composed of a of butanes and iso-i' and normal butyl eneis', such as that obtained from a cracking operation. Normal 'butylenes have become increasingly important with the need for; greater quantities of butadiene for the production of synthetic rub capacity or additional purification facilities. It

is particularly important that the normal butylene subjected to dehydrogenationior production of butadiene be free from'isobutylene and isoand normal butanes not only because 'the presence of these hydrocarbons in this reaction Y 25 normal and isobutylenes and normal and isobuintroduce vdiiiiculties for the reasons mentioned. but because the isobutylene and isobutane are in themselves extremely valuable starting materials for the production of many vital products and must be conserved whenever possible.

The conventional method forrecovering normal butylene from a mixture thereof with butanes and isobutylene is to subject the mixture to a'cold acid extraction process, by which is meant the contacting of the hydrocarbon mixture with about 65% sulfuric, acid at a temperaturebetween about 60 and 90 F. Sulfuric acid of this strength at these temperatures has a selective absorption action for isobutylene. The resuit is'thatdsobutyleneoi' about=97% purity-is recovered from the acid extract,-leavin'g a residsolvent power for olefins as comparedto "parafe fins. From this extraction stepv there-isrecovered a C4 hydrocarbon mixture containing about 90% of normal butylene. "In general, it

may be stated that. slightly less than halfof the normal butylenes contained in the feed stock to this conventional method is recovered, and then only in admixture with 10% of other 04 hydrocarbons. 1

According to the present invention the yield and purity of normal butylenes recovered from increased by following a different sequence of steps, including, first, a hot acid copolymerizationof the iso-- and normal butylenes contalned in-the CimiXture, followed by catalytic cracking, of the resulting copolymer and then bycold acid absorption of isobutylene from the normal v butylenes. By this procedure the yield of normal ,butylenes may be increased to better than 90%=and' the purity to a "value in the 10 neighborhood of 97%.

' Thenature of the present invention may be more clearly understood from the following detailed description of the accompanying drawing,

in whichthe single figure is a flow plan showing 18 th'enature and order of the processsing steps to which a C4 hydrocarbon mixture is subjected according to the present invention.

Referring to the drawing in detail, numeral I v designates a hot acid copolymerization unit. As 20 is known, this operation is carried out by contacting a mixture of isoand'normal butylene in the presence of 60% to 70% sulfuric acid at a teme perature between about 140 and 160 To this unit is fed a C4 hydrocarbon mixture containing tanes. This mixture may be obtained from several refinery operationsysuch as catalytic and thermal cracking, thermal polymerization and the like. Generally, the process of the present invention is applied more advantageously, relative tothe conventional procedure, to a C4 cut obtained from a thermal cracking, unit, because in this cut the content of normalbutylenes is considerably lower than in the C4 cut obtained 15 from catalytic cracking. This. C4 cut is fed .to

unitlbywayofline 2..

The acid is separated from the hydrocarbon phase in the copolymerization unit by settling, 1 the hydrocarbon phase is drawn oil, and the bulk 40 of the unreacted saturates is removedv by distil-' lation; while the heavy fraction containing the copolymer. resulting from the .;distillation operation is 'subjected tohydrolysis with dilute caustic for-the decomposition of any esters present. The

hydrolyzedcopolymerfraction is then subjected to a second distillation operation for'the production of a substantially pureco qlymer fracnon. Thesedistfllation and neutralization operations are embodied in block I of the flow plan,- 0 from which the copolymer fraction, substantially free from isoand normal butanes which are withdrawn from the unit by way of line 3. is fed by line 4 to a catalytic cracking unit I. This unit also embodies fractionation steps for the separamixtures thereof with C4 hydrocarbons may be tion of the fractions into the desired products.

The catalytic cracking is advantageously carried out by passing the copolymer through a catalyst bed comprised of attapulgus clay or natural clay which has been acid washed, such as "Super Filtrol, at from 600 to 900 F. and under 1 to- 5 atmospheres pressure, while utilizing a feed rate or a residence time of the feed stock in the reactor such that 30% to 45% of the total feed is converted per pass into a mixture which is primarily isoand normal butylene in approximately equal amounts, but also containing small quantities of hydrocarbons lighter and heavier than C4 hydrocarbons. The eiiiuent from the catalytic reactor is fractionated, the unconverted copolymer recycled to the reactor and the C4 fraction, containing isoand normal butylenes, withdrawn by way of line 6. The lighter constituents are withdrawn from the catalytic cracking unit through line I and the residues by way of line 8.

The isoand normal butylenes in line 6 are then fed to a cold acid process unit 9 of the type heretofore described. It may be observed that this unit includes not only an extraction-stage in which the sulfuric acid extracts the isobutylene from the C4 mixture, but a desorption stage in which the extracted isobutylenes are liberated from the sulfuric acid. This liberated isobutylene leaves the unit by way of line 10. A portion of'this isobutylene, to the extent that it is required to adjust the content of isobutylene in the feed stock to the hot acid process so that this stock will contain equal molecular proportions of isobutylene and normal butylenes is recycled to hot acid copolymerization unit. It may be observed here that in the event the feed stock is one which contains more isobutylene than normal butylenes, the molecular proportions'oi these constituents are adjusted by the addition of normal butylenes. The remainder of the isobutylene is withdrawn from the system.

The normal butylenes leaving the" cold acid processing unit 9 through line II will be about 97% pure. This substantially pure material is fed to a dehydrogenation unit [2 for the catalytic dehydrogenation of normal butylenes to butadiene. From this point on, the process is the same as that previously practiced, the eflluent from the dehydrogenation unit passing by way of line [3 to an extraction system.

If it be assumed that the feed to the hot acid process is composed of 17.3% isobutane, 51.2% normal butane, 8.2% isobutylene and 23.3% normal butylenes, the composition of this feed is adjusted according to the present invention by adding suflicient isobutylene to yield a resulting composition having 15% isobutane, 44.6% normal butane, 20.2% isobutylene and 20.2% normal butylenes. On the basis of a feed of 1-00 barrels, this and normal butylenes in equal amounts. Thus. thi product will contain about 20.75 barrels of each of these butylenes. From this quantity of feed of this composition to the cold acid process there may be expected a recovery of 20.2 barrels of a normal butylene cut containing 97% normal butylenes, 1% isobutylene and 2% butanes; In contrast to this result, a hundred barrels oi the same C4 cut processed according to the method formerly practiced yielded about 11.3 barrels of normal butylene fraction containing about of normal butylenes. Thus, the process of the present invention represents a considerable improvement in the recovery of normal butylenes from refinery C4 cuts containing them in admixture with isobutylene and butanes.

It will be apparent that the process of the present invention is susceptible to considerable variation without any departure from its essentialnature. For example, the copolymerization step may be conducted with difierent catalysts and under different conditions, most of which are known. Likewise; the cold acid extraction of isobutylene may be replaced by any other suitable operation for the quantitative separation of normal butylenes from isobutylene. The particular processing steps described are given for illustrative purposes merely because they represent the most conventional operations for the purposes intended and the ones most likely to be employed in the commercial practice of the process of the present invention. g

The nature and objects of the present invention having been fully described and illustrated, what I wish to claim as new and useful and to secure by Letters Patent is:

1. In a method for recovering normal butylenes from a C4 fraction containing them in admixture with isobutylene. and butanes, the steps which comprise adjusting the'composition of said C4 fraction to the extent necessary to provide in it equalmolecular proportions of isobutylene and normal butylenes, subjecting the resulting mixture to a treatment suitable for the copolymerization of normal and isobutylenes, separating the resulting copolymer from the product of this treatment, subjecting said copolymer to catalytic cracking under conditions suitable for the cracking of the copolymer into its component parts with a minimum production of. gas and coke, recovering a mixture of normal and isobutylenes, from said "cracking operation, and recovering substantially pure normal butylenes from said mixture.

2. A method according to claim 1 in which the mixture of normal and isobutylenes from the cracking operation is subjected to extraction with sulfuric acid-under conditions suitable for the preferential absorption of normal butylenes and substantially pure normal butylenes are recovered from the sulfuric acid. I

3. A method according to claim 1 in which the original C4 fraction contains butylene in excess of isobutylene, the mixture of normal and isobutylenes from the cracking operation is separated into its components and at least a part of the isobutylene is recycled to the feed to the copolymerization operation in order to adjust the ratio,-

of isobutylene to normal butylenes'in said feed.

IKE' D HALL. 

